Generally, humidity is the culprit. Dunno about Humbrol, but spraying Testors Dullcote in humid conditions gives you a very nice frosted effect.
Perhaps shaking the spray can to mix the aerosol and the varnish base also plays a part. I remember reading something about insufficient mixing of a cellulose based varnish causing a \'white powder\' effect, too, but I can\'t be sure about that.